1. Field of the Invention
The present invention relates to an image binarization system which is employed in an image reader or a facsimile machine to convert a read image into a binary-encoded image.
2. Description of the Related Art
In an image processing apparatus which makes use of image signals, binary-encoding processings are performed to convert an image read by an image reader (the image being composed of gradations determined in accordance with the image density thereof) into an image expressed by binary codes, namely black and white codes. In the binary-encoding processings, an image made up of gradations is converted into the binary codes by using a predetermined threshold value.
Normally, a plurality of threshold values are used in the binary-encoding processings performed by an image processing apparatus. To be specific, an image read by the image reader is classified into two image portions, one being a portion, such as a character or a line, exhibiting sharp contrast in relation to surrounding regions, the other having a portion composed of gradations, such as in a photograph. (The former and latter portions will hereinafter be referred to as a "character portion" and a "photograph portion", respectively.) Of these two image portions, the character portion is converted into binary codes by use of a fixed value determined beforehand, while the photograph portion is converted into binary codes by use of a pseudo gradation method, such as a dithering method. If the entire image read by the image reader were to be converted into binary codes by use of only a single, fixed, threshold value, the image quality of the character portion would not be adversely affected as a result of conversion, since its original image density would be maintained; however, the image quality of the photograph portion would be adversely affected, since use of a single threshould value would result in the loss of the original gradations. It is for this reason, that a plurality of threshold values are used in the image conversion process.
If, on the other hand, the entire image read by the image reader were converted into binary-encoded image data by use only of a synthetic dithering method, the photograph portion of the image would retain its original gradations, but the image quality of the character portion would be degraded, degrading the quality of the image as a whole. Thus, as can be clearly seen from the above, if binary-encoding processings of a single type are used in relation to images including different-feature portions, it is impossible to obtain images of satisfactory quality, as a result. This is because, as mentioned earlier, if an image is not binary-encoded in accordance with the specific features of its constituent portions, the quality of the resultant binary-encoded image as a whole will be degraded. Moreover, if, after the binary-encoding processings, the image is enlarged or reduced in scale, the image quality will be further degraded. Further, since the data compression method used in the binary-encoding of the image should be appropriate to the features of the various portions of the image, so as not to lower the efficiency of the binary-encoding processings, the original image should therefore first be divided into regions, in accordance with their features, and then binary-encoded.
A conventional image processing apparatus divides an image into a character portion and a photograph portion, and binary-encodes the divided portions in accordance with the features thereof. To divide the image, the conventional apparatus detects the maximum image density difference .DELTA.Dmax relating to a local region of the image, and compares the detected image density difference .DELTA.Dmax with a predetermined threshold value Th. Based on results of the comparison, the conventional apparatus divides the image into the character and photograph portions.
However, the above conventional apparatus may mistakenly determine that a character and a chart are both photograph portions if they are expressed with bold lines. (Characters and charts formed with bold lines will be hereinafter referred to as bold line portions.) This is because the maximum image density difference .DELTA.Dmax is small on the bold lines. If the character or chart formed with bold lines is regarded as a photograph portion, it will be subjected to the gradation-based binary-encoding processings, such as processings using a dithering method. Therefore, the bold lines of the character or chart will include a white portion after the processings, whereby the image quality will be deteriorated.
Let it be assumed that an original to be read is made up of normal-image density character region A, blurred-character region B (wherein a character does not present a sharp contrast to the surrounding regions), bold character region C, and photograph region D (wherein the image density does not change greatly), and that the dynamic range of the image density is of 8 bits (0 to 255). In this case, the maximum image density differences .DELTA.Dmax detected on the basis of a local region of 4.times.4 picture elements are as follows:
Region A: (DD to FF)h (h: hexadecimal notation) PA1 Region B: (10 to 40)h PA1 Region C: (0 to 10)h PA1 Region D: (10 to 40)h
If the criterion threshold value Th is (80)h, the maximum image density difference .DELTA.Dmax is judged in one of the following two ways: EQU .DELTA.Dmax&gt;Th . . . (1) EQU .DELTA.Dmax.ltoreq.Th . . . (2)
If regions A-D are judged on the basis of these formulas, then region A is determined as a character portion, and each of regions B-D are determined as a photograph region. Therefore, the gradation-based binary-encoding processings, such as processings using a dithering method, are performed with respect to regions B-D. Actually, however, regions B and C are character portions, so that the image quality of these regions are inevitably deteriorated as a result of the binary-encoding processings.
As mentioned above, the conventional apparatus cannot binary-encode an image in an optimal manner if the image includes a character portion, a photograph portion and a bold character portion. After the binary-encoding, therefore, a white portion is included in the bold character portion of the image.